Method for producing europium activated yttrium vanadate phosphor

ABSTRACT

A HIGHLY PURE AMMONIUM METAVANADATE WITH IMPROVED PARTICLE SIZE AND BULK DENSITY IS PRODUCED FROM AN INPURE AMMONIUM METAVANADATE BY DISSOLVING THE INPURE AMMONIUM MATAVANADATE IN WATER, MAINTAINING CONTROLLED TEMPERATURE CONDITIONS FOR A PERIOD OF TIME, FILTERING, MAINTAINING THE FILTRATE UNDER CONTROLLED PH AND TEMPERATURE CONDITIONS FOR A SPECIFIED PERIOD OF TIME, CRYSTALLIZING SOLID AMMONIUM METAVANADATE, WASHING THE CRYSTALS WITH WATER, VACUUM DRYING UNDER CONTROLLED TEMPERATURE AND HEAT TREATING UNDER AGITATION AND CONTROLLED TEMPERATURE CONDITIONS. THE METAVANADATE IS FIRED WITH A YTTRIUM AND EUROPIUM SOURCE TO PRODUCE A EUROPIUM ACTIVATED YTTRIUM VANADATE PHOSPHOR.

United States Patent 3,746,652 METHOD FOR PRODUCING EUROPIUM ACTI- VATEDYTTRIUM VANADATE PHOSPHOR James E. Mathers, Ulster, Felix F. Mikus,Towanda, and Ramon L. Yale, Ulster, Pa., assignors to GTE SylvaniaIncorporated No Drawing. Original application May 28, 1970, Ser. No.41,562, now Patent No. 3,653,816. Divided and this application Sept. 13,1971, Ser. No. 180,184

Int. Cl. C09k 1/44 US. Cl. 252-301.4 R 3 Claims ABSTRACT OF THEDISCLOSURE A highly pure ammonium metavanadate with improved particlesize and bulk density is produced from an impure ammonium metavanadateby dissolving the impure ammonium metavanadate in water, maintainingcontrolled temperature conditions for a period of time, filtering,maintaining the filtrate under controlled pH and temperature conditionsfor a specified period of time, crystallizing solid ammoniummetavanadate, washing the crystals with water, vacuum drying undercontrolled temperature and heat treating under agitation and controlledtemperature conditions. The metavanadate is fired with a yttrium andeuropium source to produce a europium activated yttrium vanadatephosphor.

CROSS REFERENCE TO RELATED APPLICATION This application is a divisionalapplication of Ser. No. 41,562, filed May 28, 1970, now Pat. No.3,653,816, which is assigned to the assignee of the present invention.

BACKGROUND OF THE INVENTION This invention relates to the production ofa highly pure ammonium metavanadate. More particularly, it relates to amethod for producing ammonium metavanadate of improved properties thatcan be used to produce an improved luminescent property.

Vanadium is generally found as an ore containing other metallicelements. Conventional purification techniques in producing commercialvanadate salts from vanadium sources do not completely remove all of theelements. Although the metals are present in the manufactured salts inrather minute amounts such as below 0.5% by weight, the impurities evenat these low levels affect the brightness of luminescent materialsproduced from a vanadate raw material source such as ammoniummetavanadate. A typical luminescent material that can be manufacturedusing ammonium metavanadate as a raw material is yttrium orthovanadateuseful as a luminescent phosphor in cathode ray tubes and as a colorcorrector in mercury vapor lamps. Typical metal impurities that can bepresent include aluminum, calcium, chromium, copper, iron, magnesium,manganese, molybdenum, lead, silicon and titanium.

Additionally, technical grade or high purity commercial grade ammoniummetavanadate generally have an average particle size greater than about25 microns and a bulk density of about 13 to 15 g./in. Smaller averageparticle sizes such as below microns and decreased bulk densities suchas below about 7 g./in. yield improved brightness in the luminescentmaterials.

It is believed, therefore, that a process that produces a highly pureammonium metavanadate having improved particle size and bulk densitythereby enabling the production of a luminescent material having ahigher brightness would be an improvement in the art.

SUMMARY OF THE INVENTION in accordance with one aspect of this inventionthere is provided a process for producing a highly pure am- 3,746,652Patented July 17, 1973 monium metavanadate having an improved particlesize and bulk density. The process comprises the following sequentialsteps. An aqueous solution consisting essentially of water and an impureammonium metavanadate having a weight ratio of ammonium metavanadate towater of from about 3:100 to about 7:100 is formed. The temperature ofthis solution is maintained at from about C. to about C. for about onehour and is then filtered. After filtration, the pH of the filtrate isadjusted to about 9 with sufficient relatively dilute ammoniumhydroxide. The solution having the adjusted pH is held at about 40 C.for about one hour under agitation; thereafter, the temperature of thepH adjusted solution is lowered to at least about 16 C. and held at thattemperature while agitating for an additional hour. Subsequently, theagitation is discontinued and the crystals that are formed are separatedfrom the liquid. After separation the crystals are washed by slurryingwith water at a temperature of about 40 C. The washed crystals are thenseparated from the water and dried at 25 C. under about 20 mm. Hgabsolute pressure. After drying, the crystals are heat treated underagitation and at a temperature of about 45 C. to about 55 C. for about24 to about 48 hours. The resulting product has a reduced metalliccontent and has an average particle size of less than about 10 micronsand a bulk density of less than about 7 grams per cubic inch.

In accordance with an additional aspect of this invention there isprovided a method for converting excess vanadium used in the manufactureof the luminescent materials into high purity ammonium metavanadate.

In accordance with a further aspect of this invention there is provideda method for producing an improved yttrium orthovanadate.

For a better understanding of the present invention, together with otherand further objects, advantages, and capabilities thereof, reference ismade to the following disclosure and appended claims in connection withthe above description of some of the aspects of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Commercial grades of ammoniummetavanadate are technical grade and high purity grade. Each of thesevanadate source materials contain metallic impurities such as aluminum,calcium, chromium, copper, iron, magnesium, manganese, molybdenum, lead,silicon and titanium. Although each source contains some metalimpurities, the high purity grade contains less impurities than thetechnical grade and has a whiter body color. In technical grade ammoniummetavanadate the maximum individual metallic impurity content isgenerally less than about 0.5% by weight and has .a body color(expressed as percent of the reflectance of magnesium oxide subjected toa 420 nm. source of light) of from about 35 to about 45. High puritygrade ammonium metavanadate generally contains a maximum individualmetallic impurity content of about 0.1% by weight, and a body color ofabout 65 to 75. In most instances the particular impurity that is at thehighest level will depend upon the source of vanadium bearing ore. Inany event, any of the commercial grades of ammonium metavanadates can beappreciably up-graded by the practice of this invention.

An impure ammonium metavanadate source material, such as that previouslydescribed, and water are added together and the temperature adjusted toabout 95 C. A weight ratio of metavanadate to water of from about 3:100to about 7:100 can be used with a weight ratio of 5:100 being preferred.Lower temperatures can be used, however, the concentration of ammoniummetavanadate will be decreased. Higher temperatures tend to decomposeammonium metavanadate, therefore, to achieve practical operating resultsit is preferred to form a solution having a weight ratio of ammoniummetavanadate to water of about 5:100 so that the benefits of higherconcentrations can be achieved without having insoluble materialpresent.

The solution that is formed is held at about 95 C. for about 1 hour andthen filtered to remove any insoluble material that is present.

The filtrate, while under agitation, is cooled to about 40 C. and the pHis adjusted to about 9.0 by the addition of sufficient dilute ammoniumhydroxide. By dilute ammonium hydroxide it is meant an aqueous ammoniumhydroxide solution having a concentration of ammonium hydroxide of fromabout 2% to about 5% by weight. Although ammonium hydroxide solutionshaving concentration greater than about 5% by weight can be used,accurate pH control is more difiicult. If solutions more dilute thanabout 2% by weight are used, the concentration of vanadate in thesolution becomes too low for practical operating conditions. Agitationis generally protitled with conventional agitators used to provideagitation to solutions havin the viscosity of about that of water. Thedegree of agitation need only to be sutlicient to achieve gooddistribution of the components of the solution. The type of agitationand degree of agitation will be dependent upon the type of vessel used,availability of equipment and other factors that will be obvious o thosefamiliar with chemical processing equipment design.

After the filtrate is maintained at about 40 C. and the pH is maintainedat about 9 for about one hour, the temperature of the solution islowered to at least about 16 C. while under agitation. The lowertemperature and agitation is maintained for about 1 hour and then theagitation is discontinued. The resulting crystals are separated from theliquid phase. Generally, settling and decantation of the clearsupernatant liquid is the practical method for separating the crystalsfrom the supernatant liquid. The crystals are thereafter washed byslurrying in water having a temperature of about 40 C. Water having atemperature above about 40 C. dissolves too much of the vanadate andwater having a temperature much below about 40 C. yields inadequatepurification. The crystals are separated and dried at about 25 C. in avacuum drier having an absolute pressure of about 20 mm. Hg.

The dried crystals are thereafter heated under agitation to atemperature of from about 48 C. to about 50 C. for about 24 to about 48hours to thereby produce an ammonium metavanadate having a bulk densityof about 5-6 grams/in. and a white body color of about 85-95% of MgOwhen subjected to 420 nm. light.

In the manufacture of vanadate phosphors excess vanadium is used andthereafter is removed by washing the fired phosphor with sodiumhydroxide. The vanadium is generally in the form of V About 1.5 parts ofammonium chloride is added for every 1 part of V 0 in the caustic washsolution. The solution is held at about 25 C. for about 30 minutes whileunder agitation. The agitation is discontinued and the mixture isfiltered and the crystals are washed with an ammonium chloride solutioncontaining about 5% ammonium chloride washed with water having atemperature of about 25 C. The resulting ammonium metavanadate crystalscan be purified and improved by the process of this invention.

To more fully illustrate the subject invention, the following detailednon-limiting examples are presented. All parts, proportions, andpercentages are by weight unless otherwise indicated.

Example 1 About 5 parts of an impure ammonium metavanadate is dissolvedin about 100 parts of water having a temperature of about 95 C. Theammonium vanadate has an average particlesize of about 30.5 microns(FSSS). The

following impurities are analyzed as being present by emissionspectroscopy.

flectance of MgO when subjected to 420 nm. excitation. After thesolution is formed, the temperature is maintained at about 95 C. forabout one hour. The hot solution is thereafter filtered and the filtrateis cooled to 40 C. The pH of the cooled filtrate is adjusted to about 9with about 20 parts of an aqueous solution of ammonium hydroxide havinga concentration of about 2.9% by weight. Agitation is continued and thetemperature is maintained at about 40 C. for about one hour. Thereafter,the ammonium metavanadate solution is cooled to about 16 C. withagitation. After the temperature has been at 16 C. for about one hourthe agitation is discontinued and the crystals are separated and washedwith about 3 parts of water having a temperature of about 40 C. The wet,washed crystals are dried under vacuum at about 20 mm. Hg and atemperature of about 25 C. for about 24 hours. The dried crystals areblended and heated during the blending to a temperature of about 4850 C.for about 24 to 48 hours.

The resultant ammonium metavandate has a bulk density of about 5 to 6grams/in. and an average particle size of about 9.6 microns (FSSS). Thebody color is white and has about 90% of the reflectance of MgO whensubjected to 420 nm. excitation.

Analysis of samples by emission spectroscopy for impurities indicate thefollowing impurity content:

Analysis of the mother liquor by emission spectroscopy from thecrystallization step when concentrated by removing about of the waterindicates the following:

P.p.m. Al Less than 1. Ca 50 to 500. Cr 10 to 100. Cu Less than 1. Fe 10to 100. Mg 10 to 100. Mn 5 to 50. Mo 10 to 100. Pb Not detected. Si 50to 500. Ti 10 to 100.

The foregoing analyses indicates that the precedure removes a largeportion of the metallic impurities. It is also apparent that body color,bulk density and particle size are improved.

EXAMPLE 2 A series of samples of ammonium metavanadate are used toproduce europium activated yttrium orthovanadate phosphors by heatingammonium metavanadate and coprecipitated yttrium and europium oxalate atabout 975 C. for about 2 hours. The various europium activated yttriumorthovanadate phosphor are subjected to various ultraviolet wave lengthsas excitation media and the brightness is measured and expressed as thepercentage of a known yttrium orthovanadate standard. Table 1 gives theresults.

Sample 1 is produced from a technical grade ammonium metavanadatesupplied by Vanadium Corporation.

Sample 2 is produced from a high purity grade ammonium metavanadatesupplied by Union Carbide Company.

Sample 3 is produced from technical grade ammonium metavanadate suppliedby Vanadium Corporation.

Sample 4 is produced from a high purity grade ammonium metavanadatesupplied by Union Carbide Company.

Sample 5 is produced from an ammonium metavanadate recovered from theexcess vanadium used in producing a yttrium orthovanadate phosphor.

Sample 1a is produced from the ammonium metavanadate of Sample 1 treatedas in Example 1.

Sample 2a is produced from the ammonium metavanadate of Sample 2 treatedas in Example 1.

Sample 3a is produced from the ammonium metavanadate of Sample 3 treatedas in Example 1.

Sample 4a is produced from the ammonium metvanadate of Sample 4 treatedas in Example 1.

Sample 5a is produced from the ammonium metavanadate of Sample 5 astreated in Example 1.

Following the procedure of Example 2 except that various amounts ofexcess ammonium metavanadate are used to produce the europium activatedyttrium orthovanadate, eight samples of yttrium orthovanadate areprepared. These samples are thereafter tested for brightness whensubjected to ultraviolet and cathode ray excitation. The results areshown in Tables 2 and 3.

Samples 6, 7, 8 and 9 are produced from a high purity ammoniummatavanadate supplied by Union Carbide using the percent excess shown inTable 2 and 3. Samples 6a, 7a, 8a and 9a are produced from the sameammonium metavanadate but purified in accordance with Example 1. Thestandards used for comparison are a yttrium orthovanadate produced bythe prior art.

TABLE 2 Phosphor eom- U.V. brightness, percent position, percent; excess254 325 396 466 Nl'liVOa used run. um. um. um.

Before treatment After Treatment While there have been shown anddescribed what are at present considered the preferred embodiments ofthe invention, it will be obvious to those skilled in the art thatvarious changes and modifications may be made therein without departingfrom the scope of the invention as defined by the appended claims.

What is claimed is:

1. A process for producing a yttrium orthovanadate phosphor comprisingforming an aqueous solution consisting essentially of said impureammonium metavanadate and water in weight ratios of from about 3:100 toabout 7:100 and at a temperature of from about 90 C. to about 95 C.,maintaining said temperature for at least about one hour,

filtering said solution to remove insoluble material therefrom, addingto the resultant filtrate a suflicient amount of ammonium hydroxide toadjust the pH of said filtrate to about 9 and lowering the temperatureof said solution to about 40 C. for about one hour while underagitation, reducing the temperature of the pH adjusted filtrate to atleast about 16 C., and maintaining said temperature and agitation for atleast about one hour to form crystals of ammonium metavanadate,separating said crystals, adding at least about 50 parts by Weight ofwater, at

a temperature of about 40 (3., per part of said ammonium metavanadatecrystals and separating the resulting wet crystals of ammoniummetavanadate from said water, drying said crystals under about 20 mm. Hgof absolute pressure at about C.,

heating the resulting crystals while under agitation to a temperature ofat least about 45 C. for at least about 24 hours,

forming a mixture of said heat treated ammonium metavanadate and ayttrium and europium source, in a molar ratio of vanadate to yttrium ofabout 1.5:1 to about 1.8:1 and heating said mixture for at least about 2hours at about 975 C. to form said yttrium orthovanadate phosphor.

2. A process according to claim 1 wherein said weight ratio of impureammonium metavandate to water in said 65 aqueous solution is about5:100.

3. A process according to claim 2 wherein said temperatureof saidaqueous solution is about 95 C.

I Reterences Cited UNITED STATES PATENTS 3,653,816 4/1972 Mathers et al.23-19 V 3,667,901 6/1972 Krylov et a1. 252--301.4 R

ROBERT D. EDMONDS, Primary Examiner

